1. Field of the Invention
The present invention relates to an image processing apparatus capable of appropriately binary encoding and processing the entered image signals in accordance with the content of the image.
2. Description of the Prior Art
It has already been proposed to photoelectrically read an original image with an image sensor such as a charge-coupled device or an amorphous silicon sensor, binary encoding the thus produced signals according to a suitable level, and reproducing the original image by means of a binary dot printer such as a laser beam printer or an ink jet printer. It is also proposed to compress thus digitized signals, for example with a modified Huffman encoding circuit, for the purpose of transmission or storage in a magnetic or optical memory.
Binary encoding of the original image can be achieved either through comparison of the signals obtained by reading said original image with a fixed threshold value, or by a so-called dither method in which pixels in a predetermined pixel block, for example of 4.times.4 pixels, are respectively compared with plural threshold values of different densities and pseudo tonal reproduction is achieved by the number of pixels identified as black or white in said block.
The latter dither method is effective since tonal image reproduction can be achieved with a relatively simple structure. However, the dither method is associated with the drawback of loss of resolving power in the reproduced image. On the other hand, the former binary digitizing method is incapable of tonal reproduction, although the resolving power is sufficiently high.
Consequently, in order to achieve tonal image reproduction while maintaining the resolving power, it is proposed to divide the original image into a tonal reproduction area and a high resolution reproduction area and to manually or automatically switch the method of binary digitizing so as to apply the dither method to the former area and to apply the binary digitizing with a fixed threshold value to the latter area.
In order to identify the content of an image on the original in an automatic and real-time manner, there is already known a method of utilizing the difference between the maximum and minimum image densities in an image area corresponding to the dither matrix used in the dither method, containing plural threshold levels, based on the fact that the change in image density is slow in a tonal image but is steep in a high resolving image. This method is effective as it can be conducted with a relatively simple structure composed of memories of several lines in the main scanning direction, a comparator and a subtracter. The image is identified as a linetone image or a character area if said density difference exceeds a determined value, which is usually an intermediate value between black and white, so that the binary digitizing with a fixed threshold value (hereinafter called binary slicing) is selected. On the other hand, in the case that the density difference does not exceed the determined value, the image is identified as a tonal image such as a photograph, so that the dither method is selected for binary digitizing. Consequently an area which is almost uniformly black or white is identified as a tonal image because of the low density difference and is binary digitized by the dither method.
The image reproduced from such binary digitized image data with a dot printer such as a laser beam printer is not disagreeable since each image block of 4.times.4 pixels contains only one or two pixels identified as black or white by the dither method.
However, in case of transmission or storage in a page memory after compression encoding, for example with the modified Huffman process, such binary image data results in a very low compression rate, giving rise to a long transmission time or an inefficient image storage with a large memory capacity.